1. Field of the Invention
The present invention relates to a method for coiling a wire around a slotless stator core of a motor.
2. Description of the Prior Art
Components of various precision apparatuses, such as optical and electronic apparatuses, require working accuracy of the nanometer order to meet the requirement for the development of higher-accuracy, higher-density, and higher-integration versions. A very high resolution is expected of a machine tool, stepper,. electron-beam exposure system, etc. that are used to work these high-precision components. In general, these machining and manufacturing apparatuses are provided with a positioning device. In many cases, the position control of the positioning device is effected by means of a rotary servomotor or linear motor that is controlled by means of a CNC. In order to increase the working accuracy of the components, therefore, the rotary servomotor or linear motor must be controlled with high accuracy.
FIG. 10 is a schematic sectional view for illustrating a magnetic circuit of a conventional servomotor. In this servomotor, slots 24 are formed in a stator core 21, which extends about a rotor 25. A winding 23 formed of a coiled wire 22 is embedded in each slot 24, whereby a magnetic circuit is formed. In this magnetic circuit, lines 27 of magnetic force pass through tooth portions 26 between the slots 24. Thus, the lines 27 of magnetic force extend depending on the location and shape of the tooth portions 26, and are not influenced by the way of coiling the winding 23.
Usually, the rotary servomotor or linear motor is subject to torque ripples, which must be minimized in order to control the motor with high accuracy.
Torque ripples can be divided broadly into two categories; one based on structural condition and the other based on electromagnetic condition. In the case of the rotary servomotor, for example, frictional resistance produced in a bearing for the shaft of a rotor can be a torque ripple based on the structural condition. On the other hand, magnetostriction caused between the rotor and a stator can be a torque ripple based on the electromagnetic condition.
Conventionally, in order to restrain the occurrence of torque ripples based on the structural condition, a proposal is made to reduce frictional resistance by supporting the shaft in a noncontact manner by means of a pneumatic or magnetic bearing. Further, use of a slotless stator core is proposed to restrain the occurrence of torque ripples based on an electromagnetic condition.
In order to form a winding by coiling a wire around a stator core, in general, slots are formed in the stator core. The slots may produce cogging torque. In the case of the stator core having slots, electromagnetic action on the rotor depends on the slot shape, and is influenced little by the wire coiling mode. Accordingly, it is necessary only that the number of turns of the winding be equal to a set number, and the position and shape of the winding are not very important factors.
In the case of the motor that uses the slotless stator core, on the other hand, the positional accuracy and shape of the winding constitute essential factors that determine the electromagnetic action on the rotor.
FIG. 11 is a schematic sectional view for illustrating a magnetic circuit of a slotless motor, and FIG. 12 is a schematic sectional view for illustrating positional errors of a winding.
As shown in FIG. 11, a winding 33 is pasted on the inner surface of a ring-shaped stator core 31 of the slotless motor that is opposed to a rotor 35. The stator core 31 is formed having no slots or tooth portions. Since lines 37 of magnetic force are influenced by the location and the way of coiling of the winding 33, the positional accuracy of the winding 33 pasted on the stator core 31 constitutes a factor that determines the incidence of torque ripples.
As shown in FIG. 12, the positional errors of the winding include (a) misalignment between turns of the wire in each block of the winding, (b) an error in the pasting position for the wire in the circumferential direction of the stator core, and (c) an error in the pasting position for the wire in the radial direction of the stator core. In some cases, torque ripples attributable to the positional errors of the winding are greater than in the case of a motor having slots.
A method for coiling a wire around a slotless stator core has already been proposed. FIGS. 13 and 14 are views for illustrating this coiling method.
As shown in FIGS. 13 and 14, windings 52 in the form of a simple segment each are prepared in advance by coiling a wire like an array by means of a jig or the like. These segment-shaped windings 52 are pasted on a stator core 51 and then connected to one another through a connecting wire. By doing this, the positional accuracy of the windings during assembly can be improved.
According to this method for coiling the wire around the slotless stator core, however, the winding is pasted on only one side of the stator core, so that satisfactory strength cannot be obtained with ease. In the case where the entire structure is molded, therefore, the winding may come off as a molding agent undergoes cure shrinkage.
Since the segment-shaped windings overlap one another, moreover, they must have a complicated three-dimensional shape. More specifically, a first-phase winding segment 52 (e.g., segment of U-phase winding), second-phase winding segment 54 (e.g., segment of V-phase winding), and third-phase winding segment 55 (e.g., segment of W-phase winding) overlap partially one another, and are pasted continuously on the stator core 51 in the circumferential direction. In order to fix the height of a straight portion (which serves as a magnet) of each segment in the normal direction, therefore, the winding segment 52 is formed having a bent portion 53.
The bent portion 53 is a portion in which the coating of the wire can be broken most easily, and is situated corresponding to an edge portion of the stator core. Therefore, a short circuit between lines or line-to-ground fault easily occurs in this portion. Accordingly, the wire is subjected to a substantial bending stress, so that its coating may be broken, possibly causing a short circuit between lines or line-to-ground fault. Besides, it is hard to arrange the winding segments with high positional accuracy, due to positional errors attributable to differences in shape and size between the segments or errors in the normal direction caused by adhesive bonding.
As shown in FIG. 15, moreover, winding lugs 58 protrude in the axial direction of a motor shaft 56 from the stator core 51 of status 30. The lugs 58 have no electromagnetic effect on the rotor. If the number of turns increases, therefore, the motor size becomes larger, thus constituting a hindrance to miniaturization. In the example shown in FIG. 16, spot-faced grooves 59 are formed by cutting those portions of a housing 57 of a support member for the stator and the like which correspond to the lugs 58, individually. If the motor is reduced in size in this manner, its mechanical rigidity lowers inevitably.